Hellfire means you have to deal damage to yourself, something you probably want avoid when you face an aggro deck. Twisting Nether is 8 mana after all, that spell is for late game. DOOM! is bad overall, you don't need card draw as warlock + it's 10 mana + it will rotate out soon. Defile is actually a high skill card, you need to do some math if you want to clear a board full of minions. Blizzard should add more cards like this because they are the reason this game is interesting and you can't call it stupid RNG when you clear a full board with defile.

A truly well designed card, it requires some skill to play and also to play against. However, it would be better at a bit higher mana cost in my opinion. It has the potential to full clear a whole N'zoth board due some spell dmg so easily.